RF: tweak to audio recording (enable mono recording as a possibility)

- main motivation is to expose the chnls parameter, if needed.
  default remains 2 chnls (stereo), but might want 1 channel for
  smaller file sizes, or when down-sampling from 48000 to 16000 to use
  google-speech. pyo had a bug that messed with downsampling 2-channel
  data (fixed in pyo rev 1115).